1. Field of the Invention
This invention relates generally to a radio network, and, more particularly, to a segmented antenna system for an offshore radio network used in marine seismic surveying.
2. Description of the Related Art
Underwater seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying one or more seismic sources and one or more seismic sensors at predetermined locations. For example, a seismic cable including an array of seismic sensors may be deployed on the sea floor and a seismic source may be towed along the ocean's surface by a survey vessel. The seismic sources generate acoustic waves that travel to the geological formations beneath the ocean floor, where they are reflected back to the seismic sensors. The seismic sensors receive the reflected waves, which are then processed to generate seismic data Analysis of the seismic data may indicate probable locations of geological formations and hydrocarbon deposits.
Seismic surveys often use more than one survey vessel. For example, a recording vessel may be dedicated to receiving data collected by one or more survey vessels. For another example, a first survey vessel, sometimes referred to as a shooting boat, may be coupled to a seismic source that generates the acoustic signal. A second survey vessel, sometimes referred to as a recording boat, is coupled to at least one seismic sensor that receives the reflected wave. For yet another example, a deployment vessel may be used to deploy the seismic cable including one or more seismic sensors, a positioning vessel may be used to position and/or re-position the deployed cables, a source vessel may be used to tow one or more seismic sources near the deployed cables, and a recording vessel may be used to record the data. One advantage to using multiple vessels is that a given survey area may be mapped in less time than would be required if the same area was mapped by a single vessel.
When a plurality of survey vessels is used to conduct a marine seismic survey, a large volume of information may be transmitted among the survey vessels. For example, seismic data recorded and at least partially processed by a survey vessel may be transmitted to the recording vessel, where the seismic data may be stored for later processing. For another example, seismic data may be transferred between the shooting boat and the recording boat. Physically connecting the vessels, e.g. by wires or cables, is difficult, or impracticable, because of the large and variable distances separating the various vessels. Consequently, wireless data links are used to transfer data among vessels in the network. For example, radio transmitters and receivers located on the vessels are typically used to form high-speed wireless data links to transfer data between the vessels in the network.
The high-speed wireless data links are typically formed using conventional omni-directional antennas. Vessels separated by a distance larger than the range of the omni-directional antenna may not be able to exchange data via the high-speed wireless link. The range of the high-speed wireless data link may be further reduced by a number of physical effects such as “fading.” Fading of the radio signal is caused by reflection of the radio signal from the sea surface. The phase-shifted reflected signal fades out the direct signal in regions of reduced sensitivity called “dead zones” around the vessels. For example, fading of a 2.4 Ghz radio signal may create a dead zone at a range of about 9-10 kilometers.
Interference with other signals and/or noise may also reduce the range of the transmitters and/or receivers. For example, traditional high-speed wireless data links may use unlicensed Industrial, Scientific, and Medical (ISM) frequency bands. The unlicensed ISM bands may also be used by other transmitters, such as those on board other ships in the vicinity of the survey vessels. The signals broadcast by the other transmitters may interfere with the high-speed wireless data link and degrade the quality of the connection. The interference may corrupt the transferred data and/or interrupt the transfer of data altogether. In some cases, the data corruption and/or the interruption of the data transfer may force a suspension of the seismic survey.
Rotating single-segment antennas have been used to extend the range of high-speed data links by increasing antenna sensitivity in a reduced range of angles in the direction of a target. However, the single-segment antennas suffer from at least three drawbacks. First, the position of the target must be continuously monitored. If the target is lost, the data transfer may be interrupted, and in some cases the survey may be stopped, while the target is re-acquired. This problem is exacerbated in marine seismic surveys that use rapidly moving survey vessels, which may also be carried by unpredictable water currents. Second, rotating single segment antennas have large numbers of moving parts, which may reduce the operational lifetime of the rotating single-segment antenna and increase maintenance costs and downtime. Third, the rotating single-segment antenna can only acquire a single target at a time.